1. Field of the Invention
The present invention relates to photocrosslinkable, organic solvent-soluble polymers having a Tg greater than 200.degree. C. In another aspect, it relates to negative-working, photocrosslinkable polymers that can be coated on a variety of supports to form useful elements such as solid-state color imaging devices having color filter arrays.
2. Description Relative to the Prior Art
Photofabrication of various devices using light-sensitive polymers is well-known in the art. Typical devices made using photofabrication techniques include cathode-ray screens having filter elements in a light-absorbing matrix, integrated circuits, various semiconductor devices and the like. The environment in which many of these devices are made or used frequently exposes the device to high temperature. Unfortunately, known light-sensitive polymers can be adversely affected by these high temperatures.
One method of making a color cathode-ray tube screen is described in U.S. Pat. No. 3,884,695. In this method, a negative, photosensitive resist composition is used in which is formed a window pattern. The pattern is then overcoated with a uniform layer of heat-formable optical filter material. Because of the thermal limitations of known photoresists, it was found necessary to protect these resists from the temperatures and materials encountered in the process with thermally and chemically inactive components. Thus, it was necessary to add to the photosensitive material additives such as aluminum silicate, zinc oxide and the like.
Non-light-sensitive, high-temperature-resistant, i.e., high-Tg, polymers are also known in the art. These polymers are used, for example, as protective coatings, as supports for elements which are used in recognized high-temperature processes such as the thermographic processes, and the like. A recent patent, U.S. Pat. No. 4,081,277, has described the use of high-temperature-resistant polymer layers which are superposed on a radiation-sensitive surface. An array of filter elements can be formed in the high-temperature-resistant polymer by heat-transferring dyes into the polymer through window areas formed in a photoresist which is coated on the polymer layer. High-temperature-resistant polymers are preferred in this process because, for best results, the dyes should be transferred at a temperature below the Tg of the polymer. Increasing the Tg of the polymers widens the selection of useful dyes.
The radiation-sensitive surface of a device as described in U.S. Pat. No. 4,081,277 preferably comprises a semiconductive wafer surface having thereon an array of photosensors. A color filter array which is formed in the heat-resistant polymer using the heat-transferable dyes is in microregistration with the array of photosensors. The surface also contains bonding pad areas which are those areas where it is desired to form electrical contacts to the device. It is readily apparent that the presence of the high-temperature-resistant dye-receiving polymers over the bonding pad areas will interfere with the electrical connections. One method of uncovering the bonding pad areas is to apply a photoresist to the polymer layer; expose the photoresist and develop it so as to form window areas corresponding to the bonding pad areas; and then attempt to etch away the high-temperature-resistant polymer using a suitable solvent through these window areas. Unfortunately, this turns out to be quite difficult because the solvent tends to undermine the photoresist and etch away areas of the polymer layer where the color filter elements have been or will be formed. In order to etch only those areas above the bonding pads, it is necessary to control the etching extremely carefully by careful control of the solvent concentration and other factors. This adds yet another critical step to an already complex process.
It is well-known that crosslinking of polymers decreases their permeability to liquids and gases. (See, for example, "The Permeability of Polymers to Gases, Vapors, and Liquids", R. W. Richards, National Technical Information Services, U.S. Dept. of Commerce, Technical Report No. 135 (AD-767-627), pp. 16-17 (1973).) Therefore, one would expect that rendering a polymer crosslinkable would also destroy its usefulness as a dye receiver in the process of U.S. Pat. No. 4,081,277 because its permeability to dye would be reduced. There is no reference which provides a photocrosslinkable high-temperature-resistant polymer, and certainty not such a polymer which is dyeable.